Intaglio inks and methods of intaglio printing utilizing these inks are well known in the art.
Intaglio printing typically refers to a method of printing on a substrate using a plate wherein the plate contains sunken areas which carry ink that is transferred to the surface of the substrate thereby forming the printed image. The plates may be engraved by machine, by hand or may be etched through a chemical process. The intaglio printing process may take the form of a sheet fed process using engraved plates or it may be a high speed roto press operation using curved etched plates as in the publication of periodicals and catalogs.
Currency is typically printed in an intaglio process using curved intaglio plates on rotary presses. Currency includes bank notes, stamps, certificates, fine line documents and similar printed substrates. The images are primarily line images and the inks used to print currency are of high viscosity. In a typical intaglio printing process, a plate containing the sunken image areas is inked by a high pressure roller system which presses ink into the engravings, etchings, cavities, etc. Next the excess ink is wiped from the flat or planar areas of the plate surrounding the various indentations, cavities, etchings, engravings, etc., by a wiper system. One wiping system comprises a clean vinyl roller which contacts the plate and wipes away the excess ink on the non-image areas of the plate in a continuous manner. The vinyl roller is then brushed with an aqueous cleaning solution to remove the ink and then dried prior to re-contacting the intaglio plate. This process is generally called "water-wipe" intaglio printing.
Another method of wiping intaglio plates comprises using disposable fibrous material to remove the excess ink from the non-image areas of the plate. This results in massive quantities of ink impregnated waste to dispose of representing potential environmental hazards. Yet another method of removing ink is the use of a doctor blade to scrape ink from the non-image areas of the intaglio plate. The doctor blade system is used in publication gravure processes. Optionally, the plate is inked and wiped several times in succession.
Next the inked and wiped plate is brought into contact under high pressure with a substrate and the ink is transferred from the image areas of the plate to the substrate. Then the plate is again inked, surface wiped and printed in a continuous manner. The printed substrate is removed from the press and dried.
Intaglio inks conventionally used in the art for printing currency are typically high solvent content inks containing solvents such as aromatic hydrocarbons, normal parafins, isoparafins, and cycloparafins. The use of these solvent based inks in an intaglio printing process results in currency which has a striking appearance, and, the printed image is resistant to flexing and to water. However, fibrous wiping systems must be used to wipe intaglio plates.
As previously mentioned, the used, fibrous wiping materials represent environmentally hazardous waste which must be handled and disposed of in an appropriate manner. In addition, the conventional high solvent content inks are known to result in unacceptably high levels of organic solvent vapors being discharged to the environment.
There is a continual search in this art for printing inks which have good printing characteristics and produce a satisfactory printed substrate but which reduce or eliminate hydrocarbon solvents and hazardous waste which is generated as a result of using these inks. It is particularly preferred to eliminate or minimize hazardous waste generated from the intaglio printing process by using intaglio inks which are known as "water-wipe" inks. However, there are numerous problems associated with the use of water-wipe intaglio printing inks including variations in drying rate and printing characteristics and, more specifically, the fact that water-wipe printing inks tend to have poor resistance to water when printed upon a substrate and dried. In addition, water wipe printing inks typically have a high organic solvent content, typically about 15.0 wt. % to about 35.0 wt. %.
Accordingly, what is needed in this art are water wipe printing inks having a low organic solvent content useful in an intaglio printing process which have, when printed upon a substrate and dried, good flexibility, good appearance and excellent resistance to water.